1. Field of the Invention
The present invention relates to a monitoring device which measures the urinary volume in urinary bladder to inform beforehand any possibility of urinary incontinence.
2. Description of the Related Arts
A monitoring device of the kind stated above is known from Japanese Patent Laid-Open No. 63-311952. This known device has a function to measure the positions of the walls of the urinary bladder from the time difference between transmitted and received supersonic waves. When the urinary volume increases in the urinary bladder, the diameter of the bladder increases. It is therefore possible to detect the timing desired to urinate by sensing increase in the above-mentioned time difference.
In another known device, a high-frequency electric current is supplied to flow between a pair of terminals set on the surface of a human body. Another pair of sensor electrodes are disposed in the path of the electrical current. High-frequency voltage signal is picked up by the sensor electrodes. The level of the high-frequency signal is lowered when electrical impedance is decreased due to an increase in the urinary volume in the body. It is thus possible to detect the urinary volume.
The first-mentioned method relies upon detection of displacement of the urinary bladder wall. The displacement of the urinary bladder wall, however, does not properly indicate the urinary volume because the urinary bladder muscle exhibits a complicated behavior in accordance with the increase in the urinary volume. This method, therefore, does not provide high accuracy of detection when displacement of a specific portion of the bladder wall alone is detected. Referring to the second-mentioned method relying upon change in electrical impedance, it is true that the impedance changes in close correlation to the change in the urinary volume. Actually, however, the electrical impedance varies also depending on other factors of the human body. This method, therefore, cannot provide a satisfactorily high accuracy of detection if the detection is conducted by employing specific impedance value as the threshold.